US3115393A - Process for boron production - Google Patents

Description

Dec. 24, 1963 w. 1 RoBB PROCESS FOR BORON PRODUCTION Filed April 25. 1961 /n Vemor WU/fef L. Robb United States Patent Oitice 3,115,393 PEQCESS FR BGRGN PRDUC'HUN Walter L. Robb, Scotia, NSY., assigner to General Electric Company, a corporation or New York Filed Apr. 25, i963., Ser. No. 95,470 6 Claims. (Cl. 232i9) This invention relates to a process of making highpurity elemental boron. More particularly, the invention is concerned with a process which comprises contacting an inert surface with a volatilized mixture of a boron hydride and a boron halide selected from the class consisting of BCL, and BBr3 (hereinafter referred to as boron halide) at a temperature at least as high as the decomposition temperature of the boron hydride, the said inert surface being substantially inert to the deposited boren, the b ron hydride and the boron halide.

Although silicon and germanium in high-purity state are now used Widely as semiconductor compositions, nevertheless, boron is now beginning to be recognized as a material, which in the highly purified state may also iind many applicaitons in the semiconductor art. However, the means for obtaining integral high purity boron from boron halid-es are not as satisfactory as those available for obtaining high-purity silicon or germanium.

Unexpectedly, I have discovered a readily useful process for obtaining elemental pure boron from relatively inexpensive materials. in general, my process for mal:- ing this high-purity boron involves decomposition of a boron halide of the above class in the presence of a boron hydride. Boron trichloride and boron tribrornide are relatively cheap (in contrast to boron hydrides) and the ability to decompose a boron halide, particularly at relatively low temperatures makes this process attractive for making boron of high-purity.

Difficulty has been encountered in the past in effecting the decomposition of a boron halide, such as boron trichloride, and boron tribromide. This is due to the fact that in order to effect decomposition of the boron halide, in the presence of hydrogen as the reducing agent rcquired to give the elemental boron of high-purity (hereinafter referred to as pure boron), elevated temperatures on the order of llOG to 1268" C. are generally required. Still higher temperatures are required for the deposition ol boron triiiuoride. Although boron trii tide can he decomposed at reasonable temperatures, it is, however, thc most expensive oi the boron halides, and is the most diihcult to purify to the Ctate required for boron deposition useful in semiconductor applications. As far as is known, for optimum semiconduoting properties, the boron is preferably a low temperature (about 800 to 1000" C.) rhoinbohederal crystal structure (known as red boron) and to obtain such boron the temperature at which the boron halide is decomposed must be within the range of about 80G to 1G69" C. Although one can deposit the desired boron readily b-y the decomposition of boron hydrides, the starting boron hydrides are so expensive that it Would be highly desirable to find a process which would reduce the cost of the high purity boron.

UneXpectedly` l have discovered that by employing mixtures of a boron halide (both boron trichloride and boron tribromide may be used) with a boron hydride which is volatile at elevated temperatures, and if this mixture is heated or brought in contact with an inert surface 3,115,393 Patented Dec. 24, 1963 maintained at a temperature which is at least as high as the decomposition temperature of the boron hydride, it is possible to eilect decomposition of the boron halide and also of the boron hydride to form high-purity boron which is then deposited on inert surface which comes in contact with the boron being released from the decomposition of boron hydride and the boron halide. The temperatures at which this decomposition oi the boron halide can tal-1e place is much lower than the temperatures at which boron halides ordinarily decompose and is within the necessary range of about 800 to 1000" C. at which the more desirable red boron is formed.

boron hydride Which is employed in the practice ot the present invention can be either a liquid, solid or gas as long as it is capable of being volatilized at elevated temperatures so as to decompose in the presence of the boron halide. As the boron hydride decomposes, this forms a nascent hydrogen which is highly reactive with the boron halide promptly reducing the boron halide to essentially pure boron which is then allowed to deposit on an inert surface. The decomposing boron hyd ide also deposits a proportional share of high-purity boron. lncluded in the boron hydrides which may be employed are, for instance, diborane (Bille), tetraborane (51H10), pentaborane (35H9), dihydropentaborane (BSI-lll), decaborane (BIQHM), etc.

The inert surface which is used on which to deposit the pure boron is one which is inert not only to the boron being deposited but also is inert to either the boron hydride or the boron halide. included among such inert surfaces (in tube, rod, bar, ribbon, etc., form) may be mentioned, for instance, tantalurn, titanium, zirconium, boron nitride, etc. inert surfaces offering sources of contamination, such as quartz, etc., should be avoided as the inert surface.

In carrying out my process, it is necessary to rst bring together the boron hydride and the boron halide, preferably under pressure and thereafter lead said mixture of materials in the gaseous form into a deposition chamber in which `a wire or other inert surface is maintained at a temperature above 809 C. and advantageously within tl e range of from about 800 to 1000 C. While maintaining a reduced pressure. The inert surface, advantageously in the form of a Wire or rod, is brought up to the desired temperatune by means of electrical resistance (or heated by induction, or by radiation, etc.) and is presen-t in the zone in which the mixture of the boron hydride and boron halide gases are brought together. As the mixture of the boron hydride and boron halide comtes in contact with the heated inert surface, such as the heated Wire, the boron hydride decomposes giving: oilC nascent hydrogen which, in turn, reduces the boron halide to pure boron which is then deposited on the inert surface. The process whereby the boron hydride and the boro-n halide are brought in contract fwith the heated inert sur- Inace provides ka convenient for a continuous process for imalcing pure boron and also for making thick sections `of pure boron by building up, on the inert surface, continuous layers of the boron being deposited as the mixture of the boron hydride land the boron halide comes in Contact with the heated surface.

Thereafter, the high-purity boron adhered `to the inert surace can be removed either mechanically or by means of etching or dissolving away the inert surface with a suit- 3 able acid (leg, hydrochloric, hydrolluoric, etc., acids) leaving 'behind .a hollow tube of solid high-purity boron. If one desires boron chips, the boron can easily be cracked from the inert surface leaving only a thin boride layer adherent to the filament.

A suitable appartus for carrying out the decomposition of the boron halide with the boron hydride is found in the accompanying drawing in which the single ldlGURiE shows an apparatus which can be used and actually was used in depositing highpurity boron in the manner described in the examples` which follow.

IIn .the attached figure, .the boron halide and the boron hydride are mixed together in a high pressure cylinder 1. The mixture of the boron halide and the boron hydride is lled through piping 2 in which the flow of boron halide and boron hydride lis controlled by a valve 3 into a deposition chamber 4i. The deposition chamber is advantge'ously made of Pyrex whose walls are cooled by circulating water through a surrounding .fjaclcet (not shown), this being important in order to prevent the formation of solid boron hydride polymers on the wall of the deposition chamber. Although the deposition charnber shows bringing in the mixture of boron hydride and boron halide through the bottom of the chamber S, it will be apparent that the rvapors could be brought in at either end lor at the middle of the chamber, or at several points. -By arranging the ow of the vapors of boron hydride and boron halide, and by periodically changing the direction of gas flow, rods with near uniform diameters can be produced.

yInside the deposition cell are two wire electrodes 6 formed :of an electrically conducting material, such as brass, copper, etc. These electrodes by means of electrically conducting bars 7 support the inert surface in the form lof a wire '8 which is heated to temperatures of 800 C. and higher by means of resistance heating. As the reduction `of the boron halide and the boron hydride continues, pure boron 9 will deposit on the heated wire 8. lThe volatile products and unreacted boron halide and boron hydride are then nemoved through an outlet pipe 1i) which leads into a cold trap 1l for Ithe purpose of condensing the unreacted boron hydride and boron halide. The flow of boron halide and boron hydride into the deposition chamber and the ultimate removal of volatile products including volatile reactants iis accomplished by -a vacuum pump 1.2 which is attached to the cold trap. The olw of volatile products as a result of applying a 'vacuum is controlled by means of an Alphatron gauge 1-3 shown in the attached figure.

Thereafter, having accomplis-hed the reaction between the inert heated surface and of the `mixture of the boron hydride fand the boron halide, any boron hydride or boron halide which failed to decompose or react can he readily recycled for further reaction with the heated surface. This permits a continuous process whereby full realization of yields from both the boron hydride and the boron ihalide can be realized.

lIn order that those skilled in the art can better under- :stand how .the present invention is practiced, the following :examples are given by -Way of illustration and not by way of limitation. In the examples, the apparatus used to carry out the reaction was that described in the at- Iached drawing.

Example 1 In this example, diborane and boron trichloride, in the molar natio of l mole of the former and two moles cf the latter were mixed together in the high pressure cylinder and the mixed gas was then introduced continuously for 40 minutes into the deposition cell shown in the attached drawing. The inert surface in the deposition cell was a tantalum wire about 30 mills in diameter land about 9 inches long raised to temperature by resistance heating. The wine was maintained at a temperature within the range of about 950 to 1000o C. The gas pressure in the deposition chamber was maintained at about 6() microns mercury pressure by a liquid nitrogen trap and a vacuum pump as shown in the attached drawing. The amount of boron depositing on the wire and the amount of gas leaving lthe deposition cell were measured and from this data one could calculate the amount of dlibonane and boron trichloride which decomposed to form the boron metal. These calculations revealed that for each mole of diborane and each two moles of boron trichloride used in the feed or mix, approximately 1.5 moles boron had deposited. it was further determined that52.5% of the diborane had decomposed and 24.1% of the boron trichloride had decomposed. The unreacted ingredients (after removal of any HC1) could be recycled and brought into contact again with the heated inert wire.

To exemplify what would happen if instead of using the boron hydride and the boron halide, one had used hydrogen with the boron halide, the following test was carried out.

Example 2 ln this example, the same conditions of reaction were carried out as in Example 1 with the exception that diberane was omitted and the molar ratio of boron trichloride to hydrogen in the feed was l to 1.5. The amount of hydrogen used was calculated as being comparable to that formed if all of the diborane in the Example 1 had decomposed. After carrying out the reaction similarly as in Example 1, but this time for l hour, no de-tectable amount of boron was deposited, establishing clearly that under the deposition conditions used in Example 1, molecular hydrogen was not a sufficiently strong reducing agent to reduce the boron trichloride. Even when the tantalum filament used for the purpose of depositing boron thereon was first coated with boron before contacting the latter wi-th the mixture of the boron trichloride and the hydrogen, still no additional boron was found deposited thereon. This established clearly that the boron hydride, particularly the diborane in Example 1, had an important part in the low temperature, low pressure reduction of the boron halide 'to deposit essentially pure boron.

It will, of course, be apparent to tho-se skilled in the art, that instead of using the boron trichloride or diborane employed in the foregoing examples, boron tribromide and other boron hydrides may be used, many examples of which have been given above, without departing from the scope of the invention. The inert surface can also be varied `widely as long as it does not interfere with the reduction reaction, nor does it in any -way adversely react with either the deposited boron, the boron hydride or the boron halide used. Obviously, the conditions of reaction (including the temperature conditions) may also be greatly varied, and the means of introducing the boron halide and the boron hydride may be varied within the scope of my process.

The above means for depositing boron can also be used in lining neutron counter tubes used in atomic energy applications.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The process for making readily recoverable highpurity elemental boron which comprises contacting an inert surface with a volatilized mixture of a boron hydride and a boron halide selected from the class consisting of boron trichloride and boron tribromide at a temperature at least as high as the decomposition temperature of the boron hydride, whereby elemental boron is formed both (l) through the decomposition of said boron hydride and (2) by the reduction of said bor-on halide by nascent hydrogen released by said hydride decomposition, and thereafter removing the formed boron from the inert surface.

2. The process as in claim 1 in which the boron hydride is selected from the class consisting of diborane, tetraborane, pentaborane, dihydropentabcrane, and decaborane.

3. The process for making readily recoverable highpurity elemental boron which comprises contacting an inert surface with a volatilized mixture of boron trichloride and diborane Where in the inert surface is maintained at a temperature at least Ias high as the decomposition temperature of the diborane, whereby elemental boron is formed both (1) through the decomposition of said boron hydride and (2) Aby the reduction of said boron halide by nascent hydrogen released by said hydride decomposition, and thereafter removing the boron from contact with said inert surface.

4. The process for making readily recoverable highpurity elemental boron which comprises contacting an inert surface with a volatilized mixture of boron tribromide and diborane wherein the inert surface is maintained at a temperature at least as high as the decomposition temperature of the diborane, whereby elemental References Cited in the file of this patent UNITED STATES PATENTS 2,307,005 Ruben Dec. 29, 1942 2,839,367 Stern et al June 17, 1958 2,854,353 Schwope Sept. 30, 1958

Claims (1)

1. THE PROCESS FOR MAKING READILY RECOVERABLE HIGHPURITY ELEMENTAL BORON WHICH COMPRISES CONTACTING AN INERT SURFACE WITH A VOLATILIZED MIXTURE OF A BORON HYDRIDE AND A BORON HALIDE SELECTED FROM THE CLASS CONSISTING OF BORON TRICHLORIDE AND BORON TRIBROMIDE AT A TEMPERATURE AT LEAST AS HIGH AS THE DECOMPOSITIONS TEMPERATURE OF THE BORON HYDRIDE, WHEREBY ELEMENTAL BORON IS FORMED BOTH (1) THROUGH THE DECOMPOSITION OF SAID BORON HYDRIDE AND (2) BY THE REDUCTION OF SAID BORON HALIDE BY NASCENT

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